Conventionally, the printing of a continuous slip has been carried out by high-speed business form-printing rotary presses.
Such a business form-printing rotary press is quite large in size and requires much time for exchanging printing patterns. Further, during the time when necessary adjustments are made for the pattern exchange, a large loss of the printing sheet is produced. To overcome these difficulties, various handy printing machines or presses have been proposed, particularly for the printing of a small lot of a continuous slip.
Generally, in such continuous slip printing machines, a continuous slip sheet is supplied between a pair of opposed, rotating blanket and impression cylinders, and is intermittently fed so as to effect the printing. Japanese Laid-Open (Kokai) Patent Application No. 62-64552 discloses one such known printing method in which a print sheet is intermittently fed by a pair of tractors disposed respectively on the sheet supply and discharge sides of the impression cylinder and a suction conveyor. Another such conventional method is shown in FIG. 1 in which a pair of tractors b and b, disposed respectively on the sheet supply and discharge sides of an impression cylinder a, feed a printing sheet c so as to effect the printing.
Generally, the conventional printing machines or presses under consideration are so designed that the impression cylinder a is movable relative to a blanket or rubber cylinder d between an operative or printing position where the former is disposed in opposed, contiguous relation to thelatter, and an inoperative or non-printing position, where the former is spaced apart from the latter. With such a construction, when the impression cylinder a is in its inoperative position, the printing sheet c is loosened, and therefore a slack develops on the printing sheet.
Even in the operative position of the impression cylinder a, since the impression cylinder a has a recess or reduced-diameter portion e formed in a peripheral surface thereof, a slack also develops on the printing sheet c when the recess e is brought into opposed relation to the blanket cylinder d.
Therefore, when the printing machine is operated either in the inoperative position, or in the operative position when the impression cylinder a with the recess e is disposed in opposed relation to the blanket cylinder d, the printing sheet c is liable to flatter. As a result, the ink on the blanket cylinder d is transferred to the printing sheet c to produce smudges thereof. The printed sheet with such smudges has no commercial value.
Further, the slack of the printing sheet adversely affects the register (pre-register) of an image relative to the printing sheet and therefore an oblique image is produced.
To overcome the above-mentioned deficiencies of the prior art, there has been proposed a method of eliminating such a slack of the printing sheet. As shown in FIG. 1, in this method, a roll f is provided between the tractor b on the sheet supply side and the impression cylinder a is disposed on an upper side of the printing sheet c. The roll f is brought into contact with the upper side of the printing sheet c under the influence of gravity so as to eliminate the slack of the printing sheet c by the weight of the roll f and associated parts.
With the above slack-eliminating method, however, it is necessary to lock the roll f in its upper inoperative position when the printing sheet c is to be set on the pair of tractors b and b so that the roll f will not interfere with the setting of the printing sheet. This is a rather cumbersome operation. In addition, since the roll f is designed to contact the printing surface of the printing sheet c, the printing surface is liable to be damaged by the roll f, which adversely affects the quality of the printed material. Another disadvantage is that when the printing sheet c is thin, the perforations of the printing sheet fitted on feed pins of the tractor b are liable to be deformed and enlarged by these feed pins if the weight of the roll f and associated parts are too heavy. On the other hand, if such weights are too light, the slack of the printing sheet c cannot be eliminated satisfactorily, particularly where the printing sheet c employed is thick.